In order to extract oil efficiently from certain oil fields, in particular those which contain viscous oil or bitumen deposits, steam is sometimes used usually with the primary purpose of increasing the temperature of the deposit (thereby lowering its viscosity), in large part by transferring heat as the steam condenses. Generally, steam is introduced though an ‘injection’ well shaft, and the heated deposit is removed via a ‘production’ well shaft.
As will be familiar to the skilled person, there are various steam stimulation techniques. For example, in Steam Assisted Gravity Draining (SAGD), when a reservoir containing a viscous resource deposit has been identified and geology allows, two bores are drilled, both with horizontal sections in the reservoir, an upper shaft running above a lower shaft. To allow thick, tar-like resources to flow, steam is injected through the upper shaft (and also, in some wells, initially through the lower shaft) causing the resource to heat up, liquefy and drain down into the area of the lower ‘production’ shaft, from which it is removed.
Other related techniques are ‘steam flooding’ (also known as ‘continuous steam injection’), in which steam is introduced into the reservoir though (usually) several injection well shafts, lowing the viscosity, and also, as the steam condenses to water, driving the oil towards a production well shaft. In a variant of this, so-called cyclic steam injection, the same shaft may function both as an injection well shaft and as a production well shaft. First, steam is introduced (this stage can continue for a number of weeks), then the well is shut in, or sealed, allowing the steam to condense and transfer its heat to the deposit. Next, the well is re-opened and oil is extracted until production slows down as the oil cools. The process may then be repeated.
In some instances steam injection techniques may be applied to existing wells that were not originally steam assisted to improve and/or maintain production beyond which could be achieved in the absence of steam stimulation.
Steam injection may be achieved in various ways depending on the type of well and steam assistance being employed. For example some conventional steam injection well shaft casings typically included a long slot from which the steam is released in order to achieve even heating of the reservoir. However, as the steam tends to follow the path of least resistance within the reservoir, heating can be localised. This meant that the so-called ‘steam cavern’ or ‘steam chamber’ formed could be irregular in shape, leading to inefficient production and the risk of ‘steam breakthrough’ whereby steam finds its way to the production well, mixing with the oil as it is extracted.
More recently injection well casings have been designed with number of discrete vents with slide valves rather than single long slots. Examples are described in WO2012/082488 and WO2013/032687 in the name of Halliburton, which also produces a commercial product known as the sSteam™ Valve. Such valves may be selectively controlled, based for example on an estimation of the shape of the steam chamber, to try to improve the shape by selective injection of steam along the length of an injection well shaft.
For the various steam assisted approaches it would be beneficial to be able to monitor the characteristics of the steam injection. This may be useful simply for providing information about the overall effect on the reservoir but in some applications it may be possible to control the steam injection, i.e. vary the overall flow rate or pressure or selectively control individual valves along the length of the injection well so as to achieve a desired profile.